Cellulose fibers are favored by people due to characteristics of renewable raw material, excellent performance, extensive application and so on. However, processing technologies of cellulose fiber are still dominated by the traditional viscose method at present. The pollution to environments caused by the processes during production is gradually taken into account by people, thus, a green and environment-friendly processing technology is urgently needed to be developed. In the 90s of last century, a processing technology, which prepares a stock solution by directly dissolving cellulose with an organic solvent, i.e., N-methylmorpholine-N-oxide (NMMO) and then carries out spinning, thoroughly solves the problem of environmental pollution of the viscose method. And fibers prepared by this technology are named Lyocell fibers by BISFA (The International Bureau for Standardization of Man-made Fibers).
During the process of preparing Lyocell fibers through dissolving cellulose with an NMMO aqueous solution, the swelling and dissolving processes of cellulose by NMMO and effects are directly related to stability of spinning and performances of final products. And the concentration of a spinning solution and whether a technical process is energy-saving or not are directly related to the economic benefit of the process technology. Given the importance of the stock solution for spinning during the preparation of the lyocell fibers, an energy-saving and efficient method for preparing a well-distributed high-concentration cellulose stock solution for spinning is urgently needed.
During the preparation of a cellulose solution, several stages, i.e., mixing, swelling and dissolving of pulp and NMMO are required to be carried out in different devices. And in view of the several stages and the whole process of swelling and dissolving, different methods of swelling and dissolving and devices are described in different patents.
In preparation methods of Lyocell fiber disclosed by CN1635203A, CN1981075A and CN1635203A, pulp is all activated by activating enzyme and is mixed with NMMO and is swelled, and then is dewatered under decompression and is dissolved to obtain the solution for spinning.
A method for preparing a cellulose solution is disclosed by CN1468889A and is characterized in that a cellulose solution is prepared from cellulose powder with a diameter of not exceeding 1000 um and NMMO with a concentration of 88% through heating, mixing, kneading, dissolving and homogenizing in a twin-screw extruder.
A device for producing cellulose films and fibers and an integrated plant are disclosed by CN1157012A, wherein the preparation of a cellulose solution is carried out by two thin-film-evaporation devices which are connected in series and used for carrying out depressurizing and heating on a liquid mixture of pulp and NMMO to achieve swelling and dissolving respectively, thereby obtaining the cellulose solution.
For major existing methods for preparing a cellulose solution, pulp can be activated or non-activated. And solvents can have a high concentration or a certain concentration, and mixing, swelling and dissolving processes for the pulp and the solvent can be implemented separately or completed in the same equipment. At present, disclosed and industrially-implemented methods, such as CN1635203A, CN1981075A and CN1635203A all adopt high-concentration NMMO (80% to 86.7%) and activated pulp, and comprise the steps of carrying out mixing and swelling in mixing equipment and then carrying out dissolving by thin-film-evaporation equipment.
Although the generation of gel particles can be reduced to some extent, the activated pulp introduces a large volume of moisture, the problem of energy consumption caused by repeated evaporation is caused, and certain security risks are present in links such as evaporation, storage and delivery of high-concentration solvents. If crushed ordinary pulp and high-concentration NMMO are used and subjected to mixing, swelling and dissolving in a screw extruder, the problem of moisture introduction during activation can be prevented, but the requirements on crushed particle size are too high, large-scale implementation is difficult, and the problem of dust pollution during crushing is also difficult to avoid.
In addition, if high-concentration NMMO suitable for swelling is used, the problems of high swelling degree and relatively low liquor ratio exist during mixing and swelling, and it is difficult to prepare a high-concentration (12% to 15%) homogeneous solution for spinning. By adopting a method of mixing NMMO of a certain concentration with pulp and performing swelling and dissolving of the pulp by two thin film evaporators respectively, NMMO with a relatively low concentration is used in theory, and the problem of poor uniformity caused by insufficient liquor ratio during the preparation of a high-concentration solution for spinning can be effectively solved. However, during actual implementation of this method, problems that films are difficult to form in a liquid mixture, and a phase splitting occurs as a solid phase falls off and a liquid phase flows down along a wall are present when the liquid mixture is subjected to depressurized dewatering and swelling in a first-stage thin-film evaporation device, so that the uniformity of a pre-dissolved solution is difficult to guarantee.